1. Field of the Invention
The present invention is generally related to offshore platforms for drilling wells and particularly to underwater conductor guides used with the offshore platforms and jackets.
2. General Background
The drilling and completing of wells on offshore platforms is accomplished through large diameter steel pipes, called conductors, which are driven into the soil through guides connected to the offshore platform's jacket and deck. Conductors can also be pre-installed by exploratory drilling vessels and completed to a height of 10 to 20 feet above the seabed prior to jacket installation. The jacket is then lowered over these pre-installed conductors and then the conductors are extended to the surface after jacket installation is complete. The jacket is a tubular steel framework that serves as a pile template and conductor support and extends from the seabed to a few feet above the waterline. The deck is the platform's steel superstructure above the warterline which is connected to the piling, or jacket, and supports the drilling and producing facilities. The piling consists of steel tubes which secure the platform to the seabed and penetrate the soil up to 300-600 feet. Platform components (jacket, deck, piling, etc.) are built on land at fabrication yards as completely as possible in order to minimize the much more expensive offshore (site) construction.
Conductor guides are framed at various elevations within the jacket and deck to provide support for conductors such that the effect of the environment (waves, wind, current, etc.) can be safely withstood by the conductors and to maintain conductor alignment. Conductor guides are tubular shaped and open at each end to receive the conductor. The top of conductor guides, and in certain cases the bottom also, is flared outwardly in a bell or cone shape to aid in aligning and receiving the conductor and the protective bullet (cover) on a conductor on an existing well or jacket template. There are several types of conductor guide systems utilized in the offshore industry.
The more common system consists of guides rigidly connected to the jacket and deck framework. Conductors are placed through these guides and are designed to withstand forces resulting from the environment. The conductor guides and structural framework provide support for the conductors at various levels throughout the jacket and deck. Whenever possible, these conductor guides are fabricated and welded to the jacket and deck at the fabrication yard (on land) to minimize construction costs. However, some offshore construction of guides is necessary whenever jackets are set over existing wells.
Generally, three types of conductor guide assemblies are provided for this system. The first type are those within the horizontal framework levels of the jacket and typically consist of vertical guides made out of steel tubes welded to the horizontal jacket tubular members. The other types are located in the lower and upper levels of the deck. The lower deck level guides are similar to the jacket's, except they are connected to deck floor beams. These guides are located in line with the jacket guides. The upper deck level assembly consists of a grid of beams bolted to the permanent upper deck beams supporting removable hatches which line up with the conductor guides in the lower deck. Access is provided to the lower deck level, which is typically the conductor termination level, by removing the hatches.
The advantages and disadvantages of this system follow:
a. Conductor guides and framing are normally built within the jacket and deck during land fabrication, thereby minimizing construction costs. PA1 b. Occasionally jackets are set over existing wells. For these cases, offshore construction of guide assemblies is normally required. PA1 c. Since the conductors are exposed to environmental loads, it is usually not feasible to utilize this system in areas of extreme environmental effects, such as mudslides or ice movement.
If existing wells are present as mentioned in (b) above then it is necessary to enlarge the conductor guides of an offshore platform to accommodate removal of a protective "bullet" or to allow for misalignment of the wells. Typically conductor guides are sized to provide an annulus clearance of 1/2" between the inside of the guide and the outside of the conductor.
A larger annulus clearance creates a problem once the well is connected from the seabed to the surface. Without the usual small annulus the conductors can move with wave motion within the guide, causing potentially dangerous cracks in the well casing. To remedy this, bolted wedge plates are installed by divers at the underwater guides only, typically four to six for each conductor guide, which stop well casing movement by reducing the annulus.
The bolted wedges can not be pre-installed on the guide since they attach to the inclined surface at the top of the guides and the conductor casing could damage them as it is lowered through the guides. Additionally, divers have to install each wedge by removing them from a lowered basket, swimming back to the guide and attaching four bolts through the wedges and the conductor guide. This can be a very time consuming and costly phase of the project.
This leaves a need for conductor guides that are more readily adjustable to bring the clearance annulus to an acceptable range and that minimize the requirements for equipment, time, and diver effort.